Loud speaker



Jan. 26, 1932. R, E, THOMPSON 1,842,770

LOUD SPEAKER Filed Aug. 22. 1930 ATTORN EYS Patented Jan. 26, 1932PATENT OFFICE ROY E. THOMPSON, OF NYACK, NEW YORK LOUD SPEAKERApplication inea August 22, 1930. serial No. 477,089.

This invention relates to electrodynamic sound reproducing elements ofthe loud speaker7 variety adapted to-convert relatively largeamou-Iitsoftelectrical into acoustical en- 8 erg-y.v

primary object of the invention is to provide-a sound reproducinglelement of the type specied wherein the diaphragm is caused to vibrateasa unit, piston fashion', substantially 'y free from flexure. at allacoustical frequencies and` amplitudesv of vibration as a result of itsmode of actuation rather than toany inherent stiffness ofthe diaphragm.material. M lit has been, recognized that with the usual I* methods ofloud speaker operationv wherein the diaphragm is supported` forvibratile actuation at one point or portionV only, the diaphragmissubject during vibration to flcxure "in varying degree throughout itsradial extent dependent upon the-frequency and amplituderof vibration,and is accordingly characterizedf by vibration of individual portionsthereofi withA respect to other portions.

The object of this invention is accomplished bytheprovision of a soundreproducer of the so-call'e'd dynamic type, having a diaphragm which ismaintained` in a uniformly stretched conditionv during the applicationof the vibrational sound producing forces. Means are provided. wherebythese sound producing forces cause the diaphragm to vibrate as a wholesubstantiall without'flexure, there being1 obtained' there y virtuallycomplete freedom of vibrationof'individual portions ofthei diaphragmwith respect `to other portions resulting in vibrational patterns.

The above described diaphragm operation is obtained by the provision ofa pair of actuv ating coils bearingy the signal or sound-producingcurrents, these coils being situated in uniform magnetic fields andattached to the diaphragm, oneat the outer periphery and another knearthe center or apex. The diaphragm is `maintained in a uniformlystretchedy condition by the application of steady, .biasing currentstothe actuating coils, the relative directions of the biasing currentsbeing such asfto fonce-theV coils apart axially thereby maintaining thediaphragm at alll wl timesunder-continuous tension. The signal- Afeature of the invention consists in the provision of a circuitarrangement for applying the biasing and signaling current to theactuating coils in the manner stated.

Theinvention will be more clearly understood from the following detaileddescription and reference to the drawings of which:

Figure 1 illustrates in front elevation a cone type sound reproducer inaccordance with the present invention.

Figure 2 illustrates a section along'the line 2--2 of Figure 1; and

Figure 3 is a diagram of a wiring arrangement for applying the biasingand signaling currents to the actuating coils affixed to the diaphragm.

Like elements are designated in the several views by the same numbers.

The sound reproducer, or'loud speaker., illustrated in Figures 1 and 2comprises a diaphragm 10in the form of a conical frustum composed of a;light rvibratile material such as stiff paper, an aluminum alloy or thelike. Cylindrical flanges 11 and 12are provided respectivey at thelargerbase, or outer periphery, of the cone and" at the smaller base, near theapex. The outer circumference or periphery, is attached to" a supportingring 18 by means of light flexible supports 14 of soft leather or othermaterial having little or no stiffness, which are fastened to the dia.-phragm by eyelets and to the ringby screws. The ring 13 is mounted inpositionupon-a suitable rigid non-magnetic supporting crossmember 14,which may be attachedftoa stand or, if desired, to awall;

An annular electromagnet 16 including an iron or steelcore 17 ofU-shaped'cross section, is afxedto the supporting member 15.

The magnet core is provided with circular opposed pole pieces 18 and 19and is magn netized by steady direct current supplied over leads 21 and22 to the toroidal magnetizing coil 20. A second electromagnet 23similar to the first is attached to the central portion oi thesupporting member 15, this second magnet comprising an iron cup-shapedcore member 2a, provided with a circular pole piece 25 which is attachedto the outer circumference of the cup-shaped member. A central polepiece 26 projects from the center of the cup-shaped member, and the faceof this pele piece is opposite that of pole piece 25. A toroidal magnetcoil 27 serves to polarize the magnet thru the medium of direct currentsupplied thereto over leads 28 and 29.

Permanent magnets having no magnetizing coils could be used in place ofthe electromagnets, but the operation is more satisfactory with the useof the electromagnets due to the increased magnetic action obtainable.The cylindrical flanges 11 and 12 of the diaphragm extend within theannular spaces between the opposed pole pieces of magnets 16 and 23respectively and are symmetrically located with respect to the facesthereof. Signal current coils 30 and 31 for actuating the diaphragm arewound about the langes 11 and 12 respectively, the coils terminating inthe pairs of leads 32-33 and 3%-35 for applying the signaling andbiasing currents thereto.

For prop-er operation of the sound reproduring element, the relativedirections of current flow for the polarizing currents in the magnetcoils 2G and 27 as well as 'for the biasing and signaling currents inthe actua-ting coils 30 and 31 must be correct. Fig. 3 shows acombination whereby the correct result is obtained together with asuitable circuit arrangement for simultaneously applying the biasing andsignaling current in proper relative directions to the actuating coils.In Fig. 3 the electromagnets 2O and 27 are energized' by means not shownin such manner as to make corresponding pole pieces of each magnet oflike polarity as indicated.

The actuating coils 30 and 31 are bot-h Y wound in the same sense i. e.in accordance With a left hand spiral, and are serially connected inthis manner between the terminals of the secondary winding of an inputtransformer 36. With such connection it will be apparent that forsignaling waves impressed upon the primary winding of the inputtransformer currents will be set up in a series-aiding sense in theactuating windings 30 and 31 such as to operate them in unison in thesame direction axially and thereby to vibrate the diaphragm 10 as aunit.

In order to prevent flexure of the diaphragm vduring such vibrationsteady biasing currents are applied to the actuating coils in a parallelopposed sense such that the coils are at all times urged apart axiallyto maintain the diaphragm under continuous tension. ln the specificarrangement of Fig. 3 the biasing currents are supplied from a battery40 having its negative terminal connected to the junction between thewindings 30 and 31 and its positive terminal connected to the variablecontact 41 of a potentiometer 39, the fixed resistance of which isconnected to opposite terminals of the secondary transformer windingthru choke coils 37 and 3S respectively, the latterl offering a highimpedance to the entrance of signaling currents into the shunt path. Thedirections or' current liow for the biasing currents are correct 'formaintaining the diaphragm under tension.

In the operation of the device the superposition of the liuctuatingsignaling currents upon the steady biasing currents will at each instantcausethe resultant current in one actuating coil to decrease by the sameamount that it is increased in the other actuating coil. Thiscorrespondingly decreases the force on the one coil urging it axiallyaway from the other and at thel same time increases by the same amountthe force on the second coil urging it axially away from the first.There results therefore an unbalanced component of force which operatesto move the diaphragm bodily in the direction of the unbalanced forcecomponent while it is still maintained under tension by the balancedcomponents of the biasing currents which steadily urge the actuatingcoils apart.

The design of magnets 20 and 23 should of course be such that normallyequal biasing currents in windingsv 30 and 31 will cause each coil to beacted upon by thesame force. The potentiometer 39 is provided, however,in order to compensate for slight discrepancies in this respect as wellto adjust the axial position of the diaphragm with reference to themagnet.

Adjustment of the sliding contact 41 in one direction or the other fromthe median position will increase the biasing current in one vibratingcoil and decrease it in the other; in this Way varying oppositely themagnitudes of the electromagnetic forces acting upon the respectivecoils; This will produce a resultant axial shift of the diaphragm as awhole and thus provides a means of' properly centering the same.

What is claimed is:

1. A sound reproducer comprising a diaphragm, a. pair of coil windingsattached to said diaphragm, means for producing a uniform magnetic fieldaround each of said windings, means for impressing sound producingelectric waves upon said coils in an additive sense and means forimpressing uniform direct current upon said windings in an opposingsense.

2. A sound reproducer comprising a conical shaped diaphragm, a signalcurrent coil attached at the outer circumference of said diaphragm, asignal current coil attached at the inner circumference `of saiddiaphragm, a uniform magnetic field enveloping said outer coil and auniform magnetic field enveloping said inner coil, said signal currentcoils having applied thereto direct biasing currents so polarized withrespect to each other and to the polarity of the two magnetic fieldsthat said coils are urged in opposite axial directions whereby saiddiaphragm is maintained under continuous tensions.

3. A sound reproducer comprising a conical shaped diaphragm, a signalcurrent coil attached at thev outer circumference of said diaphragm, asignal current coil attached at the inner circumference of saiddiaphragm, a uniform magnetic field enveloping said outer coil and auniform magnetic field enveloping said inner coil, said signal currentcoils having applied thereto direct biasing cur rents so polarized withrespect to each other and to the polarity of the two magnetic fieldsthat said coils are urged in opposite axial directions whereby saiddiaphragm is maintained under continuous tensions, and means forsimultaneously applying signaling currents to said coils such that theyare actuated in the same axial directions thereby whereby said diaphragmis vibrated bodily substantially without flexure in accordance with saidsignaling currents.

4:. A sound reproducer comprising a diaf phragm in the form of afrustrum of a cone,

said frustrum having an axially extending cylindrical flange affixed ateach base, a sound-current coil wound on each flange, a magnet locatedat each flange, each of said magnets having a circular pole-piece oneach side of the associated coil whereby each of said coils is locatedin an annular magnetic field, said diaphragm being supported in positionby flexible means, means for impressing upon said coils direct biasingcurrents so polarized with respect to each other and to the polarity ofsaid magnets that said coils are repelled from each other in an axialdirection thereby maintaining said diaphragm under continuous tension,and means for impressing electric signaling waves upon said coils suchthat they are simultaneously actuated in the same axial directionwhereby said diaphragm is vibrated bodily without flexure.

5. A sound reproducer comprising a diaphragm, a pair of coils and meansfor producing a pair of uniform magnetic fields, one of said coils beingatt-ached to said. diaphragm around the outer circumference and Withinone of said fields, the other of said coils being attached to saiddiaphragm around the center and being located within the other of saidfields, means Vfor causing uniform direct current'to pass through saidcoils in such a manner as to urge said coils in opposite directions,

and means for causing alternating currents corresponding tosoundwavestopass through saidcoils such that they cooperate in lmovingsaid diaphragm lin `the same direction.

=6. A sound'reproducer according :to claim -5 `in which the means forycausing lthe altervDating currents to cooperate in Apassing' 'throughsaid coils a series connection of said vcoils with'the alternatingcurrent source, andthe means for causingthesaid direct curpass throughsaid coils comprises a npair of series connected impedsfinces shuntedacross said alternating current source and a source of direct currentconnected from a point between said iinpedances to the junction of saidcoils.

'l'. 3. sound reproducer .comprising a diaphrag n, coil winding attachedin proximity to the outer circumference of said diaphragm, a. coilwinding attached in proximity to the center of said diaphragm, means forimpressing signal currents upon said windings in such relative polaritythat said windings are actuated in the same direction to vibrate saiddiaiilirag;y i, and biasing means for pern enen y adjuk .iig the axialposition of said di;y u'agm, said biasing means being solely electrical.

S. A sound reproducer according to claim in which said means forpermanently adj listing said diaphragm comprises a source of uniformdirect current which flows in opposing directions through said windingsand a potentiometer for regulating the flow of direct current throughsaid windings.

9. An electric l sound reproducer comprising a diaph lagm vibratile tosound, signal current acti ting c ils attached adjacent the center andouter periphery thereof respectively, permanent electrical biasing meansurging said coils in opposite directions for maintaining said diaphragmunder continuous tension, and means for so applying electric signalingWaves to said coils for simultaneonslv actuating them in the samedirection to vibrate said diaphragm bodily substantially free fromflexure.

l0. An electrical sound reproducer comprising a diaphragm vibratile tosound, flexible means supporting said diaphragm in operative position,signal current actuating coils attached adjacent the center and outerperiphery thereof, means producing steady magnetic fields envelopingsaid coils adapted to produce axially directed reacting forces, meansfor applying steady polarizing cui rents to said coils adapted to repelthe same axially of said diaphragm for maintaining the same undercontinuous tension, and means for adjusting the relative intensities ofsaid polarizing currents for producing axial ad- `iustment of saiddiaphragm.

ll. An electrical sound reproducer comprising a diaphragm vibratile tosound, flexible means supporting said diaphragm in opera-tive position,signal current actuating coils attached adjacent the center and outerperiphery thereof, means producing steady magnetic fields envelopingsaid coils adapted to produce axially directed reacting forces, meansfor applying steady polarizing currents to said coils adapted to repelthe same axially of said diaphragm for maintaining the same undercontinuous tension, means for adjusting the relative intensities of saidpolarizing' currents for producing axial adjustment of said diaphragm,and means for applying electric Vsignaling Waves to said coils w forsimilarly and simultaneously actuating same whereby said diaphragmvibrates bodily substantially free from ieXure.

In testimony whereof I aiiX my signature.

`ROY E. THOMPSON.

